This invention relates to a method and apparatus for providing continuous wave laser activity by purely chemical means. Gaseous chlorine dioxide and gaseous nitric oxide are reacted in a system of flowing gases, to provide atomic chlorine. Gaseous molecular bromine is introduced into this system of flowing gases and reacts with the atomic chlorine to produce atomic bromine. Further, gaseous hydrogen iodide is introduced into the system of flowing gases to react with the atomic bromine, thereby generating vibrationally excited hydrogen bromide (HBr). The hydrogen bromide may be lased directly at approximately 4 .mu.m, or alternatively, the HBr may be used to transfer energy to another gaseous medium, such as carbon dioxide, to cause subsequent lasing of that gaseous medium. Transverse flow laser apparatus adapted to implement the chemical reactions and systems described herein makes up another aspect of the present invention.